Most of Canada from N British Columbia to Labrador, excluding Newfoundland; south in the Appalachians to N Georgia and NW South Carolina (Laerm et al., 1995), in the Great Plains to N Iowa, and in the Rockies to C New Mexico and EC Arizona, USA; extralimital isolates in NW and E Pennsylvania and S New Jersey.

Close relationship to, but genetic segregation from, M. rutilus supported by allozymic data (Mezhzherin and Serbenyuk, 1992; Nadler et al., 1978), although some had earlier suggested, without presentation of data, that gapperi and rutilus are conspecific (Bee and Hall, 1956; Youngman, 1975). In laboratory crosses with Eurasian M. glareolus, the partially infertile hybrids led Grant (1974) to view the two as semispecies of recent divergence. In a narrower taxonomic survey, Nadler et al. (1978) viewed Old World rufocanus as closely related to the New World gapperirutilus complex, but broader analysis of allozyme polymorphisms indicated the gapperirutilus clade to share common ancestry with glareoluscentralis, not rufocanus (Mezhzherin and Serbenyuk, 1992).

Most highly variable in gastric morphology among species of Myodes studied by Carleton (1981) and differing from M. rutilus. See MacDonald and Cook (1996:579) for the need to clarify specific and intraspecific identities of populations occurring in the Alexander Archipelago and Alaska Panhandle. Genetic structuring and heterozygosity patterns studied in populations isolated in the S Appalachians by Reese et al. (2001). See account of M. californicus for allocation of occidentalis and caurinus to M. gapperi. See Merritt (1981, Mammalian Species, 146).